1. Field of the Invention
The present invention relates to a method of manufacturing a storage disc comprising an integrated substrate formed by bonding two substrates in at least one of which information is stored.
2. Prior Art
Computers, especially personal computers are remarkably spread recently, and a storage media to be used thereby, particularly capacity of a storage disc has been high dense and kinds of the storage media are increased. There is a magnetic disc, an optical disc (e.g., CD, CD-ROM), an optical magnetic disc (e.g., MO) as the storage disc. The demand of the optical disc as the storage disc is increased recently.
Exemplifying an optical disc called a DVD, a resin substrate, that is, a single plate constituting the DVD is required to have a thickness of 0.6 mm and an outer diameter of 120 mm and an inner diameter of its central hole of 15 mm. If such a disc-shaped resin substrate is formed of a single disc-shaped resin substrate, it is low in mechanical strength and is easily deformable. Accordingly, the disc-shaped resin substrates each having the same thickness (0.6 mm) are bonded to each other to form an integrated substrate. For example, FIG. 7 schematically generally shows a DVD of the optical disc formed of two disc-shaped resin substrates (a first disc-shaped resin substrate D1 and second disc-shaped resin substrate D2) which are bonded to each other.
In FIG. 7(A), the first disc-shaped resin substrate D1 is formed by a transparent polycarbonate resin by e.g., an injection molding machine. Irregular signal pits (signal holes) serving as information signals such as sound are transferred to one surface of the first disc-shaped resin substrate D1. A reflection coating D11 (e.g., aluminum film, etc.) is formed on the signal pits, and a protection coating D12 is formed on the reflection coating D11 to prevent the signal surface from being damaged.
The second disc-shaped resin substrate D2 formed of the transparent polycarbonate resin, etc. which is same as the first disc-shaped resin substrate D1 is bonded onto the first disc-shaped resin substrate D1 in which the signals or information is stored by way of an adhesive R.
The second disc-shaped resin substrate D2 may be directly bonded onto the first disc-shaped resin substrate D1 by way of the adhesive R without forming the protection coating D12 on the first disc-shaped resin substrate D1.
The optical disc D formed by bonding two first and second disc-shaped resin substrates D1 and D2 receives light reflected on the reflection coating D11 using laser beam by a light detector, not shown, to thereby reproduce the signal.
It is needless to say that the optical disc D called the DVD can employ the second disc-shaped resin substrate D2 in which signals or information is stored.
Since the high density storage disc including the DVD not used by the single substrate or plate but it is frequently used by the plural substrates or plates, the first disc-shaped resin substrate D1 and the second disc-shaped resin substrate D2 are needed to be bonded to each other.
A series of steps are performed to form an integrated storage disc (i.e., an optical disc) by bonding each single plate.
Step 1: an ultraviolet curing resin serving as an adhesive is coated onto an entire surface of the first disc-shaped resin substrate D1.
Step 2: the second disc-shaped resin substrate D2 is placed or overlaid on the first disc-shaped resin substrate D1.
Step 3: the ultraviolet curing resin interposed between the first and second disc-shaped resin substrates D1 and D2 is developed.
Step 4: the thus developed ultraviolet curing resin is cured.
Briefly explaining the steps set forth above, first in step 1, the ultraviolet curing resin is discharged on the first disc-shaped resin substrate D1 so as to be coated therewith in the step of coating with the ultraviolet curing resin, wherein the first disc-shaped resin substrate D1 has the signal pits coated with the reflection film and the protection film.
In step 2, a transparent second disc-shaped resin substrate D2 is placed on the first disc-shaped resin substrate D1 which is coated with the ultraviolet curing resin.
In step 3, ultraviolet curing resin interposed between the first and second disc-shaped resin substrates D1 and D2 is developed to be uniformly extended between the first and second disc-shaped resin substrates D1 and D2. This development of the ultraviolet curing resin is performed by rotating the integrated disc-shaped resin substrate formed by bonding the first and second disc-shaped resin substrates.
In step 4, the integrated first and second resin substrate is irradiated with ultraviolet from the upper portion thereof, thereby curing the ultraviolet curing resin.
The integrated storage disc is manufactured by bonding the first and second disc-shaped resin substrates. In the method of manufacturing the storage disc comprising the first and second bonded disc-shaped resin substrates, since the ultraviolet curing resin is used as an adhesive, it is an indispensable condition that the ultraviolet curing resin is irradiated with ultraviolet.
However, development of material of storage disc capable of storing higher density information has been recently needed in haste. Among such material, there appears such a material which is adapted for the high density storage disc but is difficult to allow ultraviolet to pass therethrough.
Accordingly, in the aforementioned step 4, there occurs a phenomenon that ultraviolet does not pass through the storage disc to sufficiently cure the ultraviolet curing resin even if the ultraviolet curing resin is irradiated with ultraviolet for curing the ultraviolet curing resin.
The fact that ultraviolet does not pass through the storage disc means that the ultraviolet curing resin is not irradiated with ultraviolet. There is such a limit of light transparency in the curing method using the ultraviolet curing resin. Under the background set forth above, the present invention has been made.